1. Field of the Invention
The present invention relates to an optical communication system, more particularly to an optical communication system for performing communications by multiplexing plural optical signals having mutually different wavelengths, as well as an optical communication apparatus and an optical communication method used for the optical communication system.
2. Description of the Related Art
With an increasing demand for the Internet, etc., optical communication systems with greater communication capacity are needed. In order to cope with such a demand, wavelength division multiplexing (WDM) optical communication systems are widely used. Those optical communication systems are capable of expanding the communication capacity significantly with use of one optical fiber as a transmission line. In the case of such a WDM optical communication system, plural optical transmitters are used to transmit optical signals having mutually different wavelengths therefrom. Those optical signals, each having a wavelength different from those of others, are multiplexed by a wavelength division multiplexer and then transmitted to a transmission line. As those optical transmitters, wavelength-fixed light sources in which transmission wavelengths are fixed individually can be used. In addition, by using a wavelength-tunable light source in which transmission wavelength can be changed freely, those transmission wavelengths may be controlled individually. The latter case can build a flexible system. However, the wavelength division multiplexer to which plural optical transmitters are connected can handle only input/output-enabled wavelengths determined individually by each of the connected ports. That is why it has been required conventionally to manually set a transmission wavelength of each wavelength-tunable light source connected to each port.
In order to solve the above problem, for example, a configuration as disclosed in reference document 1 (Japanese Patent Application Laid-Open Publication No. 2005-277686, particularly, FIGS. 2 and 9) is proposed. The WDM optical transmission system includes plural wavelength-variable optical transmitters, an optical multiplexer, and a return optical signal generating unit. The plural wavelength-variable optical transmitters have wavelength-tunable light sources respectively. The optical multiplexer multiplexes plural optical signals, having wavelengths which are different from each other, input from the plural transmitters respectively and thereby outputs the multiplexed optical signal. The return optical signal generating unit generates a return optical signal according to an optical signal output from the optical multiplexer, and then transmits the return optical signal to the wavelength-variable optical transmitter through the optical multiplexer.
The wavelength-variable optical transmitter includes a detector that detects the return optical signal, and a controller that controls a transmission wavelength of the optical signal transmitted from each wavelength-tunable light source based on detecting the return optical signal. The controller controls so that the transmission wavelength of the optical signal transmitted from the wavelength-tunable light source matches with the port wavelength specific to an input port of the optical multiplexer.
However, such a WDM optical transmission system has been confronted with the following problems. When a new wavelength-variable optical transmitter is installed while the system is operating, an optical signal transmitted from an operating wavelength-variable optical transmitter is output thorough an optical multiplexer. Thus the optical signal transmitted from the existing transmitter is combined with an optical signal transmitted from the new transmitter in the optical multiplexer, thereby a combined signal is output. As a result, it becomes difficult to identify the optical signal transmitted from the new transmitter and generate a return optical signal according to the optical signal. In order to solve this problem, it is required to stop the system once to set a transmission wavelength for the new transmitter as mentioned above or detect the level of each wavelength with use of a wavelength level detector or the like. If the system is stopped once, another problem will arise from the service operation. On the other hand, such a wavelength level detector is usually expensive. Therefore, if such a wavelength level detector is installed, a further problem that raises the cost will arise.
In particular, if plural wavelength-variable optical transmitters are installed simultaneously, such a system will come to be confronted with the following problems. As shown in FIG. 9 of the reference document 1, upon simultaneously installing plural wavelength-variable optical transmitters, it is assumed that any one of the transmission wavelengths will match with a target port wavelength. In this case, a light emission diode (LED) emits light that covers all the subject wavelengths as return light. Therefore, the return light is detected not only by the matching wavelength-variable optical transmitter, but also by not-matching wavelength-variable optical transmitters. As a result, it is difficult to identify a wavelength-variable optical transmitter having the transmission wavelength matching with the subject port wavelength only by detecting the presence of the return light. That is why such a judgment is done according to whether a frequency of the detected return light matches with a frequency superimposed on the transmitted optical signal. If matching, it denotes that the transmission wavelength of the subject optical transmitter matches with the target port wavelength. However, synchronous detection is required for the judgment in that case. Therefore, it requires an expensive detector not shown in FIG. 9. As a result, this causes a problem of raising cost.
Furthermore, it is assumed that the transmission wavelength of any wavelength-variable optical transmitter will not match with the target port wavelength. In this case, if an attempt is made to set a wavelength for any one of wavelength-variable optical transmitters, the wavelength setting causes a problem of influencing wavelength setting for other transmitters. More specifically, while an optical signal according to a detected return light is transmitted under wavelength setting for a wavelength-variable optical transmitter, wavelength setting for other wavelength-variable transmitters cannot be performed.
Furthermore, the WDM optical transmission system as described above causes a problem that it takes a long time until the transmission wavelength is set for each wavelength-tunable light source. More specifically, this WDM optical transmission system sets a given transmission wavelength for the subject wavelength-tunable light source, and then transmits an optical signal having the given transmission wavelength after the subject wavelength-variable optical transmitter is connected to an optical multiplexer. If receiving no return light corresponding to the optical signal of the subject transmission wavelength, the system changes the transmission wavelength to another, and then transmits the optical signal again. The system repeats this operation until the wavelength controller receives a detection signal from a photodiode.
Finally, when the wavelength controller receives a detection signal, the transmission wavelength of the wavelength-tunable light source is fixed at the set wavelength. In such a way, the system increases the necessary steps in proportion to the number of wavelengths in use. In particular, in a system that makes wavelength multiplexing many times, it takes a long time until the transmission wavelength setting is completed. And accordingly, the user is required to wait long until the user is allowed to use the system.